Approximately thirty species of gymonosperms, the so-called softwoods, comprise the great bulk of the commercially important timber species useful for construction lumber. Among these are the pines which include loblolly pine (Pinus taeda), slash pine (Pinus elliotii), longleaf pine (Pinus palustris), shortleaf pine (Pinus echinata), ponderosa pine (Pinus ponderosa), red pine (Pinus resinosa), jack pine (Pinus banksiana), Eastern white pine (Pinus strobus), Western white pine (Pinus monticola), sugar pine (Pinus lambertiana), lodgepole pine (Pinus contorta); Douglas-fir (Pseudotsuga menziesii); Western hemlock (Tsuga canadensis); Sitka spruce (Picea glauca); redwood (Sequoia sempervirens); the true firs, including silver fir (Abies amabilis), grand fir (Abies grandis), noble fir (Abies procera), white fir (Abies concolor), balsam fir (Abies balsamea); and the cedars which include Western red cedar (Thuja plicata), incense cedar (Libocedrus decurrens), Port Orford cedar (Chamaecyparis lawsoniona), and Alaska yellow-cedar (Chamaecyparis nootkatensis), and Western larch (Laryx occidentalis).
Though not inclusive of all of the commercially important softwood species, the aforementioned group of conifers does include those pines which are generally considered to be commercially significant and which are or are becoming subject to intensive silvicultural management. Among these commercially significant pines, ponderosa pine, Western hemlock, Douglas-fir, and the four so-called southern yellow pines, slash, longleaf, shortleaf, and loblolly, are particularly important. Of this last group, loblolly pine and Douglas-fir have been the subject of intensive tree improvement breeding programs
Loblolly pine and Douglas-fir, like many desirable species of trees, produce good seed crops only at infrequent and undependable intervals, and good cone crops typically occur only every five to seven years. In the normal course of events, a loblolly pine seedling produces male and female flowers when it is about 11 to 16 years old. When it does that, pollen from other trees will fertilize the female flowers, which will then produce seeds. About two years later, the seeds can be harvested and used to generate new plants. While the tree can pollinate some of its own female flowers so that some of the seedlings produced can be quite similar to the parent, none of the seedlings produced will be genetically identical.
Initially, the production of seedlings depended on wild seed which is drawn from an enormously varied gene pool. It was not long before foresters began to recognize that some seedlings grew far better in localized environments than others. For example, in the Douglas-fir region, it was found to be important to plant seedlings at the same approximate altitude from which the seed had been obtained. Soon it was realized that many other tree characteristics were heritable and while these traits vary from species to species, among them might be mentioned growth rates, the tendency to have straight or crooked stems, wood density, and light as opposed to heavy limbs. Nursery managers began searching their forests for and collecting seeds from wild trees that possessed one or more desirable characteristics. However, depending on the species, it may take from fifteen to fifty years for a new generation to produce seeds of its own and several generations of breeding are required in order to maximize genetic improvement.
Accordingly, less time consuming methods have been sought to obtain genetically superior trees. To this end, various methods such as embryogenesis, organogenesis and vegetative propagation have been investigated.
Organogenesis includes the initiation of shoots from meristematic centers induced in cultured tissue explants and the subsequent rooting of these shoots. As the method is generally employed, a portion of a donor plant is excised, sterilized and placed on a growth medium. The tissue most commonly used is a portion of young cotyledon from newly sprouted seeds or the intact embryo dissected from a seed. A much lower degree of success has been reported when tree leaves or stem tissues are cultured.
In the embryogenesis process, a group of cells becomes organized into a bipolar embryoid which will, in a favorable environment, develop bud primordia at one end and root primordia at the opposite end. One commonly reported route to production of plantlets by embryogenesis, though not reported for softwoods, has been through suspension culture wherein groups of cells are suspended in a gently agitated liquid medium containing various plant growth hormones until bipolar embryoids are differentiated and developed. The embryoids are then placed on a nutrient medium for further development into plantlets.
Vegetative propagation has been practiced by grafting or rooting of stem cuttings and by hedging. So far as is known, severe hedging of seedlings, that is, where only one branch with minimal foliage is left at the base of the seedling, has not been reported previously. A procedure involving extensive pruning has also been practiced, but requires that enough foliage remain on the parent tree to ensure that it maintains an excellent vegetative state.
Vegetative propagation, when combined with a genetic selection program, offers advantages over selection and sexual reproduction of select trees. Whereas sexual reproduction captures only the additive genetic component of superiority among selected trees, vegetative propagation captures both the additive and non-additive genetic components of outstanding clones. Whereas sexual reproduction results in genetic recombination, and thus a high degree of variability even within single families, vegetative reproduction results in both growth uniformity and predictability of many important characteristics, both selected and unselected within each clone. Additionally, the collection of sufficiently large quantities of seed to meet planting demands may be both difficult and costly and thus the objective of planting genetically improved trees can be more rapidly achieved by expanding a limited number of genetically improved seedlings by means of vegetative propagation.
One problem encountered with vegetative propagation is that the genetically superior tree that one is trying to reproduce is often an old tree which has shown its potentialities. The growth of its organs is often reduced or controlled by the appearance of restrictions which inhibit or modify certain functions. Obtaining a new plant from such organs is difficult. This phenomenon is described under the terms of aging or maturation. The totipotentiality of the meristems of the young plant gives away to the more or less advanced specialization of the meristems of the old plant. Vegetative propagation by rooting cuttings supposes that this obstacle can be removed by restoring the totipotentiality of the meristem.
Accordingly, there exists a need for a method of asexually propagating pine trees by vegetative propagation which can be employed to produce a large number of genetically improved seedlings from selected, genetically superior trees.